Most silver halide color photographic elements form multicolor images in the element by subtractive color mixing. This involves the formation of yellow, magenta and cyan dye images by color development of imagewise exposed blue, green and red sensitive silver halide emulsion layers. Ideally, the subtractive dyes so formed should absorb radiation only in the region of the spectrum which is the complement of the region of exposure. Unfortunately, all dyes have some unwanted side absorptions. To correct for these unwanted side absorptions it is common practice for color negative photographic elements to employ one or more colored masking couplers. These couplers have a color which is similar to the unwanted side absorption of one of the dyes formed from one of the image couplers. The color of the masking coupler is destroyed in the areas of the image where the dye with unwanted side absorptions is formed. The way in which colored masking couplers are employed to correct for the unwanted side absorption is described in more detail in J. Phot. Soc. Am. 13, 94(1947), J. Opt. Soc. Am. 40, 166(1950) and J. Am. Chem. Soc. 72, 1533(1950).
A preferred class of colored masking couplers are the 4-phenylazo-5-pyrazolones which correct for the unwanted yellow side absorption of magenta dye-forming couplers. Such couplers have found widespread use in color photographic elements. It is known that certain substituents on the 4-arylazo group are useful. Included, e.g., are alkoxy, hydroxy, and carbonamido groups, usually in the para position to the azo function. It has been customary to include such substituents as will permit or indeed improve the propensity of the decoupled arylazo residue to be washed out of the film during processing. The masking coupler itself, however, typically includes a hydrophobic ballast group to confer non-diffusibility to the coupler, and such masking couplers are typically dispersed in aqueous coating solutions with high boiling permanent organic solvents, known in the art as coupler solvents, using conventional homogenization dispersion techniques. Coupler solvents are generally required to provide adequate coupler activity, but excess solvent can result in increased material loads resulting in increased photographic layer thickness, which may negatively impact the optical properties of the film. High solvent levels may also raise ecological concerns.
U.S. Pat. No. 5,622,818 discloses 4-phenylazo-5-pyrazolone masking compounds in which the phenylazo coupling-off group is 2-hydroxy-5-substituted. These couplers are advantageous with respect to other substitution patterns on conventional solvent dispersed masking couplers as they are easier to synthesize than masking compounds where the coupling-off group is 2-hydroxy-4-substituted. The ease of synthesis arises from the substitution pattern (on 2 and 5 positions) of the arylazo ring, because this pattern reduces the possibility of isomer formation during the preparation of the corresponding nitrophenol, a precursor to the phenylazo coupling-off group.
Alternatives to dispersing hydrophobic photographic couplers with high boiling solvents have been suggested. Water soluble or dispersible "Fischer-type" incorporated couplers, e.g., may be used in photographic elements, such as those described in U.S. Pat. No. 1,055,155, issued Mar. 4, 1913, and particularly non-diffusible Fischer-type couplers containing branched hydrocarbon chains, e.g., those referred to in the references cited in Frohlich et al, U.S. Pat. No. 2,376,679, issued May 22, 1945, Column 2, lines 50-60. Fischer-type couplers form self-assembled micellar aggregates in water, and may be directly incorporated in film or photographic systems without the need for a dispersion making step. Such micelle forming couplers typically comprise strong acid moieties, however, and typically interact with gelatin in coating formulations to cause high viscosities and coating defects. Additionally, such couplers may not provide desired levels of activity in comparison to conventional solvent dispersions.
Copending U.S. Ser. No. 09/105,507 filed Jun. 26, 1998 (and corresponding UK Application No. 9914155.8) describe a multilayer silver halide color photographic element comprising a support bearing a light-sensitive silver halide emulsion layer and a non-diffusible yellow-colored magenta dye-forming masking coupler of the formula ##STR3## wherein COUP is a magenta dye-forming coupler having the azo group attached to its coupling position; ARYL, represents an aromatic group, including optionally further substituted phenyl, naphthyl or heteroaryl groups; m represents an integer of from 1 to 4; each L.sup.1 represents a divalent linking group, preferably --O(CH.sub.2).sub.y --, --NHCO(CH.sub.2).sub.y --, or --NRCO(CH.sub.2).sub.y --, where R represents an alkyl or aryl group and y represents an integer from 1 to 4; each k is either 0 or 1; each L.sup.2 represents --NHSO.sub.2 --, --NHCO--, --SO.sub.2 NH--, or --CONH--, preferably at least one L.sup.2 group being --NHSO.sub.2 -- or --SO.sub.2 NH--, more preferably --NHSO.sub.2 --; each Z represents --SO.sub.3 M or --PO.sub.3 M, where M represents H or a counter ion such as Na, K, Li, or NH.sub.4 ; and n represents an integer of from 1 to 5, with the provisos that when at least one L.sup.1 or L.sup.2 group comprises an --NHSO.sub.2 -- or --SO.sub.2 NH-- group then the total number of Z group substituents on the coupler is at least 2, and when no L.sup.1 or L.sup.2 group comprises an --NHSO.sub.2 -- or --SO.sub.2 NH-- group then the total number of Z group substituents on the coupler is at least 3, and if k is 0 for a substituent on the ARYL group then the L.sup.2 group for that substituent is either --NHSO.sub.2 -- or --NHCO--. Masking couplers comprising the requisite number and types of L.sup.1, L.sup.2 and Z groups have been found to form self-assembled micellar aggregates in water, and accordingly may be directly incorporated in aqueous solutions without the need for a dispersion making step. The specific couplers disclosed in U.S. Ser. No. 09/105,507, however, have been found to demonstrate undesired fog levels in fresh photosensitive layer coatings.
It would be desirable to provide masking couplers which may be directly incorporated into an aqueous coating solution without the need for a dispersion-making step and associated organic solvents, while still providing good activity levels without the generation of undesirable levels of fog. Such couplers would enable improved photographic elements obtained by simplified photographic element manufacturing techniques and also desirably enable elements to be prepared with thinner imaging layers due to the absence of solvents otherwise needed for dispersing the couplers.